The present invention relates generally to systems for digitally processing the pulses generated in detector systems in response to absorbed radiation and, more particularly, to analog conditioning the signals in such systems prior to their digitization and subsequent digital processing in high resolution, high rate, digital spectrometers for x-rays or gamma rays.
There is a need, in very high counting rate applications such as synchrotron radiation research, for improved x-ray spectrometers. In many of these applications it is desired to detect and count x-rays of one particular energy under conditions where such x-rays of interest are greatly outnumbered by x-rays of a different but nearby energy. A typical example would include X-ray Absorption Spectroscopy (XAS) of dilute metallo-protein solutions, where elastically scattered incident x-rays (noise events) greatly outnumber the fluorescence x-rays (signal events) from the metal of interest. Since the x-ray spectrometer's total count rate capability is limited by energy resolution considerations, it spends most of its time processing noise pulses, which limits the acquisition rate of good signal pulses. Under these conditions it is advantageous to employ multiple detector systems to increase the total acquisition rate of good signals. Commercial spectrometers with 13 channels are now fairly commonly sold and many researchers are considering systems with over 100 channels. This technique is limited by several factors, including cost, lack of high count rate capability with pileup inspection, the lack of an energy resolved analysis of the spectrum seen by each detector, the practical difficulties associated with retuning the processing electronics for a large number of detector channels, and, often, the sheer bulk of the required electronics.
Cost is an important issue because of the large number of detector channels to be implemented. Typical instrumentation for a single detector channel using a high quality analog spectroscopy amplifier and energy spectrum analyzer ("multi-channel analyzer" or MCA) presently costs approximately $6,000. The cost of outfitting the desired 100 channels is thus prohibitively expensive for most researchers. Because of price and counting rate considerations, usually only an energy window analysis ("single channel analyzer" or SCA) is used, even for systems with only a few detectors.
A significant fraction of interesting synchrotron experiments also require energy analysis, however. These are typically experiments done using softer x-rays, in the region of 2000-4000 eV, where the energy resolution of even the best spectrometers is not adequate to fully resolve the signal energy of interest from the background energies. In these cases a simple SCA window cannot be set to accept only signal counts. Instead a full energy analysis is required, using peak fitting or deconvolution techniques to extract the signal peak from any nearby background peaks.
The throughput, or maximum countrate capability, of energy analyzing spectrometers is often set by the time it takes for the energy analyzer to process a pulse. During this time the system is "dead" and cannot accept other pulses. Common MCAs, particularly the low cost variety which are configured as personal computer cards, can be quite slow, usually limiting count rates to less than 50,000 per second. Because a factor of 10 increase is desired for synchrotron applications, MCAs are not usually employed and the cheaper and faster windowing SCAs are used instead.
Digital spectrometers have been viewed as a solution to both these problems since, once an x-ray signal pulse has been converted to a shaped peak by a digital filter and the peak's maximum value captured, the problem of providing the MCA function becomes easy since the captured magnitude is already a digital value and can be used directly as an address in the histogramming process required to produce a spectrum. No deadtime is introduced either, again because the value is already digital.
To inspect for pileup, the spectrometer must be able to detect the arrival times of the pulses coming from the preamplifier and then reject those that are closer together than the spectrometer's shaping time. If this is not done, such pulses are summed by the processing circuitry ("piling up"), and produce spectral distortions in the output. Because pileup occurs as the square of the input pulse rate, pileup inspection becomes a necessity when operating at the high count rates encountered in synchrotron experiments. The ability of the inspection circuitry to resolve sequential pulses in a digital system is generally limited by the sampling rate of its analog-to-digital converter (ADC), which defines the inter-sample period. As the number of sample periods between two sequential pulses decreases it becomes increasingly difficult to determine that, in fact, more than a single pulse is present. For very high data rates, then, high ADC sampling rates are required to minimize the number of undetected piled-up events.
Spectrometer bulk also becomes an issue when many detector channels are required. The conventional electronics required for a 13 element detector array alone completely fill an electronics rack. Thus considerably higher density is required if 100 element arrays are to be practically implemented, since even that single equipment rack has already begun to become unwieldy.
For these synchrotron applications, and many others as well, it would thus be advantageous to have a low cost, small volume spectrometry device capable of providing full energy analysis with good energy resolution at high count rates and be further capable of being interfaced to a computer system so that necessary tuning operations could be accomplished automatically by an appropriate program.
Spectrometers based on digital signal processing techniques have been considered as a solution to these problems as well, since digital circuitry is often both denser and cheaper than analog circuitry, because interfacing the spectrometer to a computer system would be quite natural, and, as noted above, providing MCA functionality could be accomplished, deadtime free, with little additional cost. In practice, however, practical attempts to produce low cost, digital spectrometer systems have failed. Experience shows that the preamplifier signal must be digitized with approximately 14 bits of accuracy in order to achieve acceptable energy resolution, and available ADCs which are both fast enough to give good time resolution and have this many bits are still very expensive. The only digitally based spectrometers produced to date have been approximately twice as expensive as conventional analog spectrometers.